140 INFUSORIA. PORIFERA OR SPONGES. 



the mouth, becomes most conspicuous. The alimentary par- 

 ticles introduced into the mouth commonly have to pass 

 down a short canal before they enter the general cavity of 

 the body ; and within this cavity a number of minute par- 

 ticles are commonly aggregated into a sort of little pellet, as 

 may be well seen when Infusoria are fed with carmine or 

 indigo. One after another of these pellets being thus intro- 

 duced into the interior, which is occupied by a soft sarcode, 

 each seems to push onwards its predecessors ; and a kind of 

 circulation is thus occasioned in the contents of the cavity. 

 The pellets that first entered make their way out after a time 

 (their nutritive materials having been yielded up), generally 

 by a distinct anal orifice, sometimes, however, by any part of 

 the surface indifferently, and sometimes by the mouth. 



1 35. The multiplication of Infusoria ordinarily takes place 

 by spontaneous fission, precisely after the manner of the 

 multiplication of ordinary cells ( 33). This process, under 

 favourable circumstances, may be performed with such 

 rapidity, that, according to the computation of Ehrenberg, no 

 fewer than 268 millions might be produced in a month by 

 the repeated subdivision of a single Paramecium. Sometimes, 

 instead of undergoing subdivision into two equal parts, the 

 Animalcule puts forth a bud, which gradually increases, and 

 then detaches itself from the parent stock. Whether any- 

 thing equivalent to the sexual generation of higher animals 

 occurs among Infusoria, is yet uncertain ; but recent re- 

 searches afford a probability in the affirmative. 



136. In the tribe of PORIFERA, or Sponges, we seem to 

 have the connecting link between Protozoa and Zoophytes. 

 For their animality does not lie so much in the individual 

 particles, as in those aggregations whioh begin to shadow 

 forth that distinction into organs which is carried out more 

 completely among Zoophytes : and there is a large section of 

 the last-named group, in which the polypes are connected 

 together, not by a regular stony or horny stem, but by a 

 sponge-like mass; while the extension of the fabric is provided 

 for by the budding out of this spongy portion of it, the 

 orifices of whose canals after a time become furnished with 

 polype-mouths. The true Sponge (fig. 81) consists of a fleshy 

 substance, composed of an aggregation of particles of sarcode, 

 supported upon a skeleton which usually consists of a net- 



